Declining Food Supply
Resident killer whales are salmon specialists. Studies by Fisheries and Oceans Canada show that 96% of their prey are salmon, of which three quarters are Chinook salmon. Interestingly, the Chinook is one of the least abundant salmon species in the north east Pacific. It is probably favoured by the whales due to its large size, high fat content, and year round availability in coastal waters. The chum is the second largest salmon species and is the second-most favoured by resident killer whales. Most chum predation occurs during a short period in the late summer and fall as the fish approach rivers to spawn.
A 2009 study by Dr. John Ford and colleagues found evidence that resident killer whales not only prefer Chinook, but also depend on them. Periods of unusually high mortality in all age classes of northern and southern resident killer whales coincided with annual Chinook salmon abundances well below historical averages. This was particularly evident in the mid-1990s, with Chinook abundance remaining well below average until 2002-2003. During this time, the southern resident population dropped by 17% and the northern residents by 8%.
Salmon declines likely have multiple interacting causes. Over-fishing, ocean regime shifts, climate change, coastal development, pollution and logging near spawning rivers all play a role. Much more research is needed to fully understand the factors that contribute to salmon survival and decline.
Harbour seals and sea lions are important prey of transient killer whales and are relatively abundant at present in B.C. and Washington. However, these species have declined sharply in western Alaska in the last two-three decades, which may be affecting transient killer whale populations in those areas.
As the diet of offshore killer whales is poorly understood, it is not clear whether prey depletion is a significant threat to them at this time.
Photo: Karl Solomon
Resident killer whales rely on healthy salmon populations to survive.
Photo: Lance Barrett-Lennard
Harbour seals are important prey of transient killer whales. The species is abundant in British Columbia but is depleted in much of Alaska.